ZnO structures and methods of use

ABSTRACT

ZnO structures comprising crystalline ZnO micro or nanorods and methods for making and using these ZnO structures are provided. The side surface of the central portion of each rod may comprise planes of the form {1 0 −1 0}, {0 1 −1 0}, {−1 1 0 0}, {−1 0 1 0}, {0 −1 1 0} and {1 −1 0 0}, with central edge regions including a crystallographic plane of the form {2 −1 −1 0} or {−2 1 1 0}. The tip of the rod may comprise planes of the form {1 0 −1 1} {0 1 −1 1}, {−1 1 0 1}, {−1 0 1 1}, {0 −1 1 1} and {1 −1 0 1} with tip edge regions including a crystallographic plane of the form {2 −1 −1 2} or {−2 1 1 2}. The rods may be joined at or near their bases to form the “flower-like” morphology. In an embodiment, a synthesis mixture is prepared by dissolving a zinc salt in an alcohol solvent, followed by addition of at least two additives. The zinc salt may be zinc nitrate hexahydrate, the first additive may be benzyl alcohol and the second additive may be urea.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 12/685,465filed on Jan. 11, 2010, which claims the benefit of U.S. ProvisionalApplication No. 60/143,489, filed Jan. 9, 2009, each of which is herebyincorporated by reference in its entirety to the extent not inconsistentwith the disclosure herein.

BACKGROUND

A major challenge in materials engineering is the controlled assembly ofpurposefully designed molecules or ensembles of molecules into meso-,micro-, and nanostructures to provide an increasingly precise control atmolecular levels over structure, properties and function of materials(Michal, D. W., Nature 2000, 405, 293 and Dai, Z. F., et al., Adv.Mater. 2001, 13, 1339). The controlled synthesis and characterization oflow dimensional crystalline objects is also a major objective in modernmaterials science, physics and chemistry (Polleux, J., et al., Angew.Chem. Int. Ed. 2006, 45, 261 and Angew. Chem. 2005, 118, 267). Manyresearchers have focused on the rational ways to control the shape,size, and dimensionality of nanomaterials. Self-assembly of inorganicnano building blocks into one-dimensional, two-dimensional, andthree-dimensional ordered hierarchical nanostructures are fascinatingbecause the variation of the arrangements of the building blocksprovides a method to tune the property of the material (Niederberger,M., et al., J. Am. Chem. Soc. 2002, 124, 13642; Niederberger, M., etal., Chem. Mater. 2002, 14, 4364; Niederberger, M., et al., Angew. Chem.Int. Ed. 2004, 43, 2270; Niederberger, M., et al., J. Am. Chem. Soc.2004, 126, 9120; Richards, R., et al., Angew. Chem. Int. Ed. 2006, 45,7277; Richards, R., et al., J. Phys. Chem. C 2007, 111, 12038 andRichards, R., et al., Adv. Mater. 2008, 20, 267).

ZnO is a particularly interesting oxide as an excellent optoelectronicmaterial because of its wide direct band gap and large exciton bindingenergy. It has been widely studied as catalyst support for methanolsynthesis and decomposition from industrial and experimental processes,because methanol can be used as an alternative energy source to diminishoil and gas resource, as well as a raw material for manmade hydrocarbonand their products (BASF, German Patents, 1923, 415, 686, 441, 443, 462,and 837, US Patents, 1923, U.S. Pat. Nos. 1,558,559 and 1,569,755; Sun,Q. et al., J. Catal. 1997, 167, 92 and Olah, G. A. Angew. Chem. Int. Ed.2005, 44, 2636). ZnO has been proven to be a quite complex andinteresting material with a variety of different structures. Thereforemany efforts have been exerted to prepare ZnO possessing controlledshapes and morphologies (Yu, H. et al., J. Am. Chem. Soc. 2005, 127,2378; Wu, J. J., et al., Adv. Mater. 2002, 14, 215; Tian, Z. R., et al.,J. Am. Chem. Soc. 2002, 44, 12954 and Zhang, T., et al., J. Am. Chem.Soc. 2006, 128, 10960). Various ZnO structures, such as nanocrystals,nanoparticles, nanocubes, nanowires, and nanosheets have been fabricatedsuccessfully. Each of these structures can be formed by a differentgrowth mechanism under a wide range of different thermodynamicconditions.

For instance, ZnO nanostructures have been grown directly from a solidsource, such as a Zn foil, or using a ZnO film as a nucleation centerfor the Zn atoms (Yu, H. et al., J. Am. Chem. Soc. 2005, 127, 2378). ZnOnanowires have also been grown by vapor deposition methods using metalnanoparticles as a catalyst at high and low temperatures (Wu, J. J., etal., Adv. Mater. 2002, 14, 215). Large arrays of oriented helical ZnOnanorods and columns were formed using simple citrate ions to controlthe growth behavior of the crystal (Tian, Z. R., et al., J. Am. Chem.Soc. 2002, 44, 12954). Complex and oriented ZnO nanostructures weresynthesized by taking advantage of the preferential adsorption oforganic structure-directing agents on different facets of hexagonal ZnOcrystals (Zhang, T., et al., J. Am. Chem. Soc. 2006, 128, 10960 and TianZ. R., et al., Nature Mater. 2003, 2, 821). The interests in fabricatingnew ZnO nanostructures have been steadily growing due largely to theexciting new applications (Huang, M. H., et al., Science 2001, 292, 1897and Wang, Z. L., et al., Science 2006, 312, 242), which imply theimportance of controlling size and shape in ZnO synthesis.

Perfectly ordered oxide surfaces are usually quite inert, so that theirchemical and catalytic properties are commonly attributed to thepresence of surface defects (Kovacik R., et al., Angew. Chem. Int. Ed.2007, 46, 4894). ZnO is widely used in catalysis, electrical devices,optoelectronics and pharmaceuticals, which often crucially depend on thedefect properties of this versatile material. It is becomingincreasingly established that in order to control the functionalproperties of nanoscale materials, it is necessary to control not onlytheir composition, shape and size, but also their defect structure(Spence J. C. H., Science 2003, 299, 839). To understand and to controlthe defect content of inorganic nanostructures can be seen as animportant goal (lschenko V., et al., Adv. Funct. Mater. 2005, 15, 1945).However, there is little research about the direct fabrication of ZnOstructures which has rich defects, though many efforts have been exertedto prepare ZnO possessing controlled shape and size.

Benzyl alcohol has been found to be a successful medium to tailor metaloxides with well-controlled shape, size and crystallinity underanhydrous conditions, for example, TiO₂ nanoparticles of anatase phasein the 4-8 nm size range (Niederberger, M., et al., Chem. Mate. 2002,14, 4364-4370). Vanadium oxide nanorods and tungsten oxide nanoplateletswith identical morphology (Niederberger, M. et al., J. Am. Chem. So.2002, 124, 13642) were synthesized in this medium by Stucky andco-workers from metal chloride precursors. Bimetallic oxides ofPerovskite structured BaTiO₃, BaZrO₃, LiNbO₃ (Niederberger, M., et al.,Angew. Chem. In. E. 2004, 43, 2270) and SrTiO₃, (Ba, Sr) TiO₃nanoparticles (Niederberger, M., et al., J. Am. Chem. So. 2004, 126,9120) with controlled particle size and high crystallinity have alsobeen prepared through a suggested C—C bond formation mechanism usingmetal alkoxides as the starting materials. In all of these studies, noselectivity in surface growth and no nanoscale building rods with richholes were found. A general drawback of the above sol-gel processesemploying benzyl alcohol for tailoring metal oxides with well-controlledshape, size and crystallinity, is the amorphous nature of the derivedmaterials, and the following heat treatment to induce crystallizationwhich usually leads to undesired particle morphology.

SUMMARY

In one aspect, the invention provides ZnO structures comprisingcrystalline ZnO micro or nanorods, the rods comprising or consistingessentially of characteristic crystallographic features. The ZnOstructures of the invention may be used as a catalyst for methanoldecomposition and formation at low temperature, dimethyl carbonateformation and photocatalysis. The ZnO structures of the invention mayfind application in solar cells, fuel cells, electrochemical cells,direct methanol fuel cells (DMFC), electric vehicle propulsion, and inalternative energy technologies, such as hydrogen generation or storage.The inventive ZnO structures may also find application in high densitymagnetic data storage and as a component or interconnect in nanodevices.

In an embodiment, the surface of the ZnO rods exhibits pits or holes.These pits or holes indicate defects in the crystalline ZnO structures.These pits or holes may contribute to catalytic activity of the ZnOrods. In an embodiment, the diameter or distance spanning the pit isfrom 1 to 200 nm.

In an embodiment, each ZnO rod has a central region and tip region, withthe average width of the tip region being less than that of the centralregion. The tip region may taper from the central region to the free endof the rod. The rods may have the wurtzite crystal structure. In anembodiment, the surface of the central region is characterized by a setof six crystal facets with an edge region located between each pair offacets. In an embodiment, each central facet corresponds to acrystallographic plane of the form {1 0 −1 0} {0 1 −1 0}, {−1 1 0 0},{−1 0 1 0}, {0 −1 1 0} or {1 −1 0 0} and each central edge regionincludes a crystallographic plane of the form {2 −1 −1 0} or {−2 1 1 0}.This central region structure is schematically illustrated in FIGS. 15 aand 15 b.

In an embodiment, the tip region may also be characterized by a set ofsix crystal facets with an edge region located between each pair offacets. In an embodiment, each tip facet corresponds to acrystallographic plane of the form {1 0 −1 1} {0 1 −1 1}, {−1 1 0 1},{−1 0 1 1}, {0 −1 1 1} {1 −1 0 1} and each tip edge region includes acrystallographic plane of the form {2 −1 −1 2} or {−2 1 1 2}. This tipregion structure is also schematically illustrated in FIGS. 15 a and 15b.

In an embodiment, the invention provides assemblies of ZnO structureshaving a characteristic “flower-like” morphology, as illustrated inFIGS. 2-3. At least some of the ZnO rods are connected at or near theirbases and radiate outward from this connection region. The centralportions of these rods have limited contact with each other, so thatmuch of the surface area of the rod is exposed to the environment. Theflower-like ZnO structures according to the invention have greatcommercial and technical potential.

The invention also provides ZnO structures made by the methods of theinvention. In an aspect, the invention also provides processes formaking the ZnO structures of the invention which are simple, low-costand practical, and is easy to scale up. In an embodiment, the ZnOstructures and structure assemblies are formed via a template-free,halide-free, efficient wet chemical method. Since no templates orsurfactants are used, subsequent complicated procedures of removingthose substances are not necessary. In an embodiment the ZnO structureare formed from a synthesis mixture which includes a zinc salt precursormaterial dissolved in an alcohol solvent. In an embodiment, twoadditives are also present in the synthesis mixture. The first additivemay be benzyl alcohol or a substituted benzyl alcohol, while the secondadditive may be urea or thiourea. In an embodiment, the methods of theinvention use a one-pot approach using the inexpensive precursor zincnitrate, optionally containing water, as a starting material.

Methanol can be used as an alternative energy source to diminishingpetroleum resources, as well as a raw material for manmade hydrocarbonand their products and a ‘methanol economy’ has recently been proposedby Olah as an alternative to a ‘hydrogen economy’ (Ref. 3c). One of thedevelopments necessary for the realization of a ‘methanol economy’ isthe development of catalysts capable of producing methanol (preferablyfrom CO₂) and decomposing methanol into H₂ and CO₂. Thus, these tworeactions have been widely studied in the realms of industry andacademics with ZnO supported Cu demonstrating the best results.

In another aspect, the invention provides methods for decomposition ofmethanol employing the ZnO structures and structure assemblies of theinvention. In an embodiment, methanol can be decomposed into carbonmonoxide and hydrogen. In another embodiment, methanol can be oxidizedto form carbon dioxide. In another embodiment, a mixture of carbonmonoxide and carbon dioxide can be formed over the ZnO catalysts of theinvention. In an embodiment, methanol decomposition can occur atrelatively low temperatures, such as a temperature below 200° C. or 80°C. In an embodiment, the method comprises the step of contacting a zincoxide microstructure or nanostructure of the invention with a gasincluding methanol vapor at a temperature from 25° C. to 200° C. for atime from 0.1 h to 12 h. In other embodiments, the temperature may befrom 150° C. to 200° C. or 25° C. to 150° C. and the time may be from0.1 h to 10 h.

In another aspect, the invention provides methods for hydrogenation ofcarbon dioxide, thereby producing methanol. The methods employ ZnOstructures and structure assemblies of the invention which are capableof catalyzing hydrogenation of carbon dioxide. In an embodiment,methanol formation can occur at relatively low temperatures, such as atemperature below 200° C. Carbon dioxide hydrogenation is more typicallyperformed with ZnO supported copper catalysts at 220-280° C. (see refs.3b, 13) so observation of these results for copper-free catalysts isunexpected. In an embodiment, the zinc oxide microstructures ornanostructures are contacted with a gas including carbon dioxide andhydrogen at a temperature from 160° C. to 250° C. for a time from 0.5 hto 20 h.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates scanning electron microscope (SEM) images offlower-like ZnO structure precursor crystals before calcination. Theas-synthesized flower-like structure can be seen.

FIG. 2 a-2 d illustrates field emission scanning electron microscope(FESEM) images of flower-like ZnO structure at different magnificationsafter calcination.

FIGS. 3 a and 3 b illustrate high magnification SEM images of ZnO rodsfrom aggregated flower-like ZnO structures at different magnification.

FIG. 4 a illustrates a SEM image of a ZnO rod. FIG. 4 b illustrates aselected area TEM image of the ZnO rod and FIG. 4 c illustrates a HRTEMimage of the ZnO rod. The Fast Fourier Transforms (FFTs), shown in theinsets of FIGS. 4 b and 4 c, reveal that the crystal structure iswurtzite oriented to Z=[01 10].

FIG. 5 a illustrates a TEM image of a ZnO rod showing the left edge ofthe material tilted at 2°, and FIG. 5 b illustrates a HRTEM image of theZnO left edge.

FIG. 6 a illustrates a TEM image of a ZnO rod showing the right edge ofthe material tilted at 2°, and FIG. 6 b illustrates a HRTEM image of theZnO right edge.

FIG. 7 a illustrates a SEM image of a ZnO rod and FIG. 7 b illustrates alocal selected area electron diffraction pattern of a ZnO rod. Theobserved SAED shows that the rod presents a hexagonal crystal structureoriented to the [01 10] zone axis. The growth direction of the rodcorresponds to {0001}.

FIGS. 8 a-8 c illustrate SEM images of ZnO rods and selected areaelectron diffraction pattern of the ZnO rods. The observed SAED showsthat the rods present a hexagonal crystal structure oriented to the [0110] zone axis (Z=[01 10]). The growth direction of the rod correspondsto <0001>. FIG. 8 d is a model of the selected area diffraction pattern.

FIG. 9 a is an SEM image which illustrates nine different rods analyzedby electron backscatter diffraction (EBSD). All of the rods exhibitedthe same wurtzite crystal structure. FIG. 9 b illustrates the rodlabeled 3 in the adjacent SEM image. The crystal structure is found tobe wurtzite, with [01 10] orientation. FIG. 9 c is another SEM image ofa ZnO rod analyzed by EBSD. FIGS. 9 d and 9 e relate to the analysis,with FIG. 9 e showing pole figures obtained from the analysis.

FIGS. 10 a and 10 b illustrates an SEM image of an individual 6-fold ZnOnano building rod (a), and SAED pattern (b). The edges of the rod tipcorrespond to (2 −1 −1 2) and (−2 1 1 2) planes. The measured angleresults to be 65° close to the theoretical value of 64.42°. The edgesalong the length are (−2 1 1 0) planes.

FIG. 11: trace a) illustrates the powder X-ray diffraction (XRD) patternof flower-like ZnO and trace b) illustrates the standard JCPDS #36-1451ZnO. The observed lattice spacings are in excellent agreement withwurtzite structure.

FIG. 12 illustrates a DRIFT spectrum obtained after exposing theflowerlike single crystalline ZnO to methanol vapor introduced N₂ flowat room temperature for 2 min.

FIG. 13 illustrates DRIFT spectra obtained after exposing the flowerlikesingle crystalline ZnO to methanol vapor introduced N₂ flow at roomtemperature for 2 min (a), heated to 70° C. (b), kept at 70° C. for 30min (c), oxygen introduced and kept for 5 min at 70° C. (d), and oxygenintroduced and kept for 30 min at 70° C. (e).

FIG. 14 illustrates DRIFT spectra obtained after exposing the flowerlikesingle crystalline ZnO to CO₂ and H₂ at 180° C. for 1 min (a), 10 min(b), 20 min (c), 40 min(d), 60 min (e).

FIGS. 15 a and 15 b schematically illustrate the ZnO rod geometry, withFIG. 15 b being drawn to emphasize the {2 −1 −1 0}, {2 −1 −1 2}, {−2 1 12} and {2 −1 −1 2} planes.

FIG. 16 shows an SEM image of bud-like ZnO structures obtained in theabsence of urea.

DETAILED DESCRIPTION

In one aspect, the invention provides ZnO structures in the form ofcrystalline “building blocks”. These building blocks may be singlecrystalline. In an embodiment, the building blocks are not simplyaggregates of ZnO particles in which separate particles can bedistinguished from each other. These building blocks can aggregate toform an assembly of ZnO structures. In an embodiment, the crystalstructure of each building block has six-fold symmetry.

As used herein, a material is crystalline if it displays long-rangeorder in the position and stacking sequence of the atoms. Asingle-crystalline structure displays a characteristic diffractionpattern of regular spots.

Crystallographers can use Miller-Bravais indices to identify the variouscrystal-facets or crystallographic planes in a hexagonal crystalstructure such as wurtzite. The Miller-Bravais index of a crystal planeis defined by the distance and orientation of the plane relative to aset of crystallographic axes and the point of origin.

FIG. 15 a shows a facet labeled (1 0 −1 0) in the central region. Thislabel is the Miller-Bravais index for the plane (1 0 −1 0). According tothe art, {1 0 −1 0} designates a family of planes including the (1 0 −10), (−1 1 0 0), and (1 0 −1 0) planes.

The vector [1 0 −1 0] points perpendicularly away from the (1 0 −1 0)plane, and <1 0 −1 0> designates a family of vectors including the [1 0−1 0], [(−1 1 0 0], and [1 0 −1] vectors.

In an embodiment, the ZnO microstructures take the form of crystallinebuilding blocks possessing six-fold symmetry and a wurtzite structurewith [0 1 −1 0] orientation (orientation with respect to one of exposedfacets). In an embodiment, the building blocks are wurtzite structurewith [0 1 −1 0] orientation and the growth direction of the buildingblocks corresponding to <0001>.

In an embodiment, the six edges of the hexagonal crystal structure have[−2 1 1 0] and [2 −1 −1 0] orientation alternately (orientation withrespect to exposed edge regions).

In an embodiment, the ZnO building blocks are elongated and rod-like. Inthis morphology, the length of the blocks or rods is generally greaterthan their width. In an embodiment, the blocks are microstructures,having at least one dimension less than one millimeter. In anotherembodiment, a block or a portion of a block may be a nanostructure,having at least one dimension (e.g. the width) less than 1 micron(micrometer). In an embodiment, a plurality of the blocks or blockportions are nanostructures. In an embodiment, the average length of theblocks is in the range 1 to 6 microns. In an embodiment, the averagewidth of blocks (central portion if tapered) is 0.1 to 2 microns. In anembodiment, the average length of the blocks is in the range 2-6microns, while the average width of the blocks is in the range 1-3microns. In another embodiment, the length of the blocks is from 1 to 3microns and the width of the blocks is from 0.1 to 1 micron. In anembodiment, the width at the free end of each block is less than thewidth in the central portion of the block. In an embodiment, the widthof the free end of a block may be less than one micron, so that this endis a nanostructure.

In an embodiment, the block or rod comprises a hexagonal prism centralportion and a pyramidal tip, as illustrated schematically in FIGS. 15 aand 15 b. Without wishing to be bound by any particular belief, thepyramidal tip may contribute to catalytic activity.

In an embodiment, the prisms may be bounded by non-polar {0 1 −1 0}planes with intersecting edges along {−2 1 1 0} planes while thepyramids are composed of {−1 1 0 1} surfaces with intersecting edgesalong {−2 1 1 2} planes. Without wishing to be bound by any particularbelief, the {−1 1 0 1} surfaces of the pyramids and/or the {−2 1 1 2}edges of the tip may provide catalytic activity for certain reactions.

In an embodiment, the assemblies of blocks or rods look like “bloomingflowers” as illustrated in FIGS. 2-3. At least some of the rods areconnected at or near their bases and radiate outward from thisconnection region. The central portions of these rods have limitedcontact with each other, so that much of the surface area of the rod isexposed to the environment. In an embodiment, the ZnO assemblies have a“flower-like” morphology, each assembly comprising a plurality of ZnOstructures,

In another embodiment, the assemblies look like “budding” flowers, asillustrated in FIG. 16. In this embodiment, the building blocks aresmaller and more closely interconnected.

In an embodiment, the flower-like ZnO material according to theinvention can be readily identified through a combination of the X-raydiffraction (XRD) pattern and scanning and transmission electronmicroscopy. In an embodiment, the ZnO material of the invention has thedistance of the lattice planes shown in Table 1 in high resolutiontransmission electron microscopy (HRTEM) when imaging the 6-foldbuilding rod, and has the morphology shown in the scanning electronmicroscope (SEM) images of FIG. 2 a-2 d, FIGS. 3 a and 3 b, FIG. 4 a,FIG. 7 a, FIG. 8 a-8 c, FIG. 9 a, 9 c and FIG. 10 a, the transmissionelectron microscope (TEM) images of FIG. 4 b, FIG. 5 a and FIG. 6 a, andthe high resolution transmission electron microscopy (HRTEM) images ofFIG. 4 c, FIGS. 5 b and 6 b, and the powder X-ray diffraction (XRD)pattern of FIG. 11. FIG. 1, which shows the precursor material beforecalcining, shows similar morphology to the calcined material.

TABLE 1 The index planes of flower-like ZnO structure. D observed (Å) Dcalculated (Å) Indexing 2.8397 2.8143 100 2.6067 2.6033 002 2.47252.4759 101 1.9083 1.9111 102

In an embodiment, the invention provides ZnO structures comprisingwurtizite ZnO micro or nanorods, the surface of the ZnO rods exhibitingpits or holes. In different embodiments, the surface area of the ZnOstructures is from 0.5 to 5 m²/g, or from 1 to 3 m²/g, as measured withthe BET method. In different embodiments, image analysis of the surfaceof the ZnO structures may show that on average at least 1%, 2%, 5%, 10%,15% or 25% of the surface includes these pit or hole surface defects. Insome regions of the rod, the concentration of these surface defects maybe higher.

The side surface of the central portion of the rod may comprise planesof the form {1 0 −1 0}, {0 1 −1 0}, {−1 1 0 0}, {−1 0 1 0}, {0 −1 1 0}or {1 −1 0 0}. The edge regions of the central portion of the rod mayinclude a crystallographic plane of the form {2 −1 −1 0} or {−2 1 1 0}.The tip of the rod may comprise planes of the form {1 0 −1 1} {0 1 −11}, {−1 1 0 1}, {−1 0 1 1}, {0 −1 1 1} or {1 −1 0 1}. The edge regionsof the tip of the rod may include a crystallographic plane of the form{2 −1 −1 2} or {−2 1 1 2}. Rods may be joined at or near their bases toform the “flower-like” morphology described previously.

In an embodiment, the invention provides methods for making ZnOstructures and structure assemblies. In an embodiment, a synthesismixture is prepared by dissolving a zinc salt in an alcohol solvent,followed by addition of at least two additives.

In an embodiment, the method comprises the steps of:

-   -   a. preparation of a mixture of        -   i. zinc salt or a hydrated zinc salt;        -   ii. a first additive, wherein the first additive is an            alcohol comprising a phenyl group;        -   iii. a second additive selected from the group consisting of            urea, a urea derivative, thiourea, a thiourea derivative, or            combinations thereof;        -   iv. an aliphatic alcohol solvent, the aliphatic alcohol            having from 1 to 3 carbon atoms;    -   b. heating the mixture of step a) from ambient temperature to a        first temperature between 180° C. to 200° C. and maintaining the        mixture at the first temperature for a time from 1 to 12 h;    -   c. heating the mixture of step b) to a second temperature from        240° C. to 300° C. and maintaining the mixture at the second        temperature for a time from 1 to 12 h;    -   d. removal of the alcohol solvent from the mixture of step c)        and    -   e. calcination in air of the mixture of step d).

In an embodiment, the zinc salt is selected from the group consisting ofzinc nitrate, zinc acetate, zinc citrate, zinc methacrylate, zincsulfate and zinc oxalate, hydrated forms thereof or combinationsthereof. In an embodiment, the zinc salt is zinc nitrate or hydratedzinc nitrate (e.g. zinc nitrate hexahydrate Zn(NO₃)₂.6H₂O).

In an embodiment, the alcohol solvent is selected so to be capable ofdissolving the zinc salt. In an embodiment, the alcohol is an aliphaticalcohol having from 1-3 carbon atoms. In an embodiment, the solvent maybe methanol. In an embodiment, the concentration of zinc ions in thesolution is from 0.01 mol/l to 1 mol/l.

In an embodiment, the first additive is an alcohol comprising a phenylgroup, the first additive being other than a phenol in which thehydroxyl group is directly bonded to the aromatic ring. In anembodiment, the first additive is benzyl alcohol (BA) or a 4-substitutedbenzyl alcohol or a mixture thereof. In an embodiment, the 4-substitutedbenzyl alcohol is 4-methoxy benzyl alcohol (MBA). In another embodiment,the 4-substituted benzyl alcohol is 4-nitrobenzyl alcohol (NBA). Thefirst additive may assist in controlling the morphology of the ZnOstructures. In embodiment, the ratio of BA and/or substituted BA to Znis at least two. In different embodiments, the ratio of BA and/orsubstituted BA to Zn is 2 to 6, 2 to 5, 2 to 4, or 2 to 3.

In an embodiment, the second additive is urea or a urea derivative(having the functional group RR′N—CO—NRR′) or a urea or a thioureaderivative (having the functional group RR′N—CS—NRR′). In an embodiment,the second additive is the compound urea (NH₂)₂CO. In an embodiment, thesecond additive can serve the function of producing hydroxide ions. Indifferent embodiments, the ratio of urea to Zn is from 0.1 to 1.0 orfrom 0.25 to 0.5. In embodiment, the ratio of BA and/or substituted BAto Zn is at least two and the ratio of urea to Zn is from 0.25 to 0.5.

Typically, the synthesis mixture will be maintained at a temperaturehigher than ambient temperature. The synthesis mixture may be maintainedat a temperature higher than the boiling temperature of the solution ina closed reaction vessel. In an embodiment, the mixture is maintained ata temperature from 150° C. to 300° C. for a time from 2 to 24 hours. Inanother embodiment, the mixture is heated from ambient temperature to afirst temperature from 180° C. to 200° C. over a time from 1 to 12 h andthen maintained at the first temperature for a time from 1 to 12. Themixture may then be heated to a second temperature from 240° C. to 300°C. over a time from 1 to 12 h and the mixture maintained at the secondtemperature for time from 1 to 12 h.

When the synthesis mixture is placed in a closed reaction vessel, thepressure in the vessel may be greater than one atmosphere. In anembodiment, the pressure in the vessel may be from 6000 torr to 10000torr.

In an embodiment, the pressure in the reaction vessel is sufficientlyhigh that the critical temperature and pressure of the solvent isexceeded, causing it to enter the supercritical state. The supercriticaltemperature and pressure depends on the solvent. For methanol, thecritical temperature is 512.6 K and the critical pressure is 8.09 MPa;for ethanol the critical temperature is 513.9 K and the criticalpressure is 6.14 MPa; for propyl alcohol the critical temperature is526.5 K and the critical pressure is 5.1 MPa (Eckert C et al., Nature1996 383, 313).

In an embodiment, no substrate is required for ZnO structure formation.However, in other embodiments, the structures may be grown on asubstrate capable of withstanding the synthesis process conditions. Thesubstrate may be glass or quartz or may be selected to provide aparticular surface orientation, such as a wafer of known crystalorientation, thereby influencing the growth of the ZnO nanostructures.

In an embodiment, solvent removal is accomplished by a supercriticaltreatment or drying. In an embodiment the solvent is placed in thesupercritical state in a closed reaction vessel, then solvent may bevented from the vessel

The mixture may be cooled to ambient temperature before the particlesare collected. In an embodiment, the mixture is free cooled, so thatcooling is not accelerated. In an embodiment, the free cooling processcan take from 1 to 5 h to reach ambient temperature.

In an embodiment, following solvent removal, zinc oxide is formed. Inanother embodiment, the invention provides the intermediate product ofthe above synthesis, a flower-like ZnO structure precursor, having thecrystalline nature of the desired particle morphology beforecalcination. An exemplary flower-like ZnO structure precursor beforecalcination has the scanning electron microscope (SEM) images of FIG. 1and the crystalline nature of the desired particle morphology beforecalcination. Without wishing to be bound by any particular belief, thehigh crystallinity of this intermediate is believed to allow thecalcined ZnO to maintain the flower-like structure of the as-synthesizedorganic-inorganic crystals of the ZnO precursor (before calcination).

Calcination may take place in air. In an embodiment, the calcinationtemperature is from 400° C. to 600° C. In an embodiment, the calcinationramp rate is from 3 to 10° C./min.

The invention also provides methods for decomposing methanol in whichzinc oxide structures of the invention are contacted with a gasincluding methanol vapor at ambient temperature or above. In anembodiment, the gas supplied to the zinc oxide structures is a mixtureof methanol and a carrier gas. Suitable carrier gases include, but arenot limited to nitrogen, argon or helium. The gas mixture may alsoinclude oxygen. The initial gas concentrations may be expressed in molarpercentages or ratios. In an embodiment, the initial concentration ofmethanol in the gas mixture is from 5% to 30%. In an embodiment, theinitial concentration of oxygen in the gas mixture is from 0.001% to0.5%.

The invention also provides methods for hydrogenating carbon dioxide inwhich zinc oxide structures of the invention are contacted with a gasincluding carbon dioxide and hydrogen at greater than ambienttemperature. In an embodiment, the gas supplied to the zinc oxidestructures is a mixture of carbon dioxide, hydrogen and a carrier gas.Suitable carrier gases include, but are not limited to nitrogen, argonor helium. The gas mixture may also include carbon monoxide. Indifferent embodiments, the initial concentration of carbon dioxide inthe gas mixture is from 5 to 30% or from 20% to 30%. In differentembodiments, the ratio of hydrogen to carbon dioxide is from 4:1 to2.5:1 or about 3:1. In an embodiment, the initial concentration ofcarbon monoxide in the gas mixture is from 0% to 20%.

All references throughout this application, for example patent documentsincluding issued or granted patents or equivalents; patent applicationpublications; and non-patent literature documents or other sourcematerial; are hereby incorporated by reference herein in theirentireties, as though individually incorporated by reference, to theextent each reference is at least partially not inconsistent with thedisclosure in this application (for example, a reference that ispartially inconsistent is incorporated by reference except for thepartially inconsistent portion of the reference).

All patents and publications mentioned in the specification areindicative of the levels of skill of those skilled in the art to whichthe invention pertains. References cited herein are incorporated byreference herein in their entirety to indicate the state of the art, insome cases as of their filing date, and it is intended that thisinformation can be employed herein, if needed, to exclude (for example,to disclaim) specific embodiments that are in the prior art. Forexample, when a compound is claimed, it should be understood thatcompounds known in the prior art, including certain compounds disclosedin the references disclosed herein (particularly in referenced patentdocuments), are not intended to be included in the claim.

Every formulation or combination of components described or exemplifiedcan be used to practice the invention, unless otherwise stated. Specificnames of compounds are intended to be exemplary, as it is known that oneof ordinary skill in the art can name the same compounds differently.When a compound is described herein such that a particular isomer orenantiomer of the compound is not specified, for example, in a formulaor in a chemical name, that description is intended to include eachisomers and enantiomer of the compound described individual or in anycombination. One of ordinary skill in the art will appreciate thatmethods, device elements, starting materials, and synthetic methods,other than those specifically exemplified can be employed in thepractice of the invention without resort to undue experimentation. Allart-known functional equivalents, of any such methods, device elements,starting materials, and synthetic methods, are intended to be includedin this invention. Whenever a range is given in the specification, forexample, a temperature range, a time range, or a composition range, allintermediate ranges and subranges, as well as all individual valuesincluded in the ranges given are intended to be included in thedisclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention.

In general the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. The precedingdefinitions are provided to clarify their specific use in the context ofthe invention.

EXAMPLE 1 Preparation and Characterization of ZnO Structures

Experimental Details Chemicals: Zn(NO₃)₂.6H₂O (98%), urea (98%),anhydrous benzyl alcohol (99.8%), and anhydrous methanol (99.8%) wereobtained from Sigma-Aldrich Chemical Incorporation and used withoutfurther purification. NanoActive ZnO was received from NanoScaleCorporation. The high purity N₂ and the mixture of O₂ and N₂ (20.5% O₂and 79.5% N₂) were supplied by General Air Service & Supply, USA.

Preparation of flowerlike single crystalline ZnO: In a typicalpreparation, 18 g of Zn(NO₃)₂.6H₂O was dissolved in 200 ml absolutemethanol. After the Zn(NO₃)₂.6H₂O totally dissolved, 1.8 g urea and 13 gbenzyl alcohol was added to the mixture in the ratioZn(NO₃)₂.6H₂O:urea:benzyl alcohol=1:0.5:2 (molar ratio). After stirringfor 1 h, the mixture solution was transferred to an autoclave and thereaction mixture was purged with 7500 torr Ar 5 times, and then apressure of 7500 torr Ar was imposed before initiating heating. Themixture was heated to 200° C. for 5 h, then to 265° C. and maintained atthat temperature for 1.5 h; finally, the vapor inside was vented(thereby removing the solvent in the supercritical state). A dry greypowder was collected and subsequently calcined with a ramp rate of 3°C./min to 500° C., then maintained at 500° C. for 6 h. The powderproduced from this preparation contains flower-like ZnO structurepossessing 6-fold building blocks and wurtzite structure with [01 10]orientation. The diameters of these 6-fold building blocks are ca. 1-3μm, and the lengths are ca. 2-6 μm.

As another example, 18 g of Zn(NO₃)₂.6H₂O was dissolved in 200 mlabsolute methanol. After the Zn(NO₃)₂.6H₂O totally dissolved, 0.9 g ureaand 13 g benzyl alcohol was added to the mixture in the ratioZn:urea:BA=1:0.25:2 (molar ratio). After stirring for 1 h, the mixturesolution was transferred to an autoclave. The autoclave containing thereaction mixture was purged with 10 bar (7500 torr) Ar 5 times, and thena pressure of 10 bar (7500 torr) Ar was imposed before heating starts.The mixture was heated to 200° C. for 5 h, then heated to 265° C. andmaintained at that temperature for 1.5 h, at last, the vapour inside wasvented. A dry grey powder was collected and subsequently calcined with aramp rate of 3° C./min to 500° C., then maintained at 500° C. for 6 h.The powder produced from this preparation contains numerous flower-likeaggregates of holes rich ZnO structures. The diameters of these 6-foldbuilding blocks are ca. 0.2-1 μm, and the lengths are ca. 1-3 μm.

In another example, 18 g of Zn(NO₃)₂.6H₂O was dissolved in 200 mlabsolute methanol. After the Zn(NO₃)₂.6H₂O dissolved completely, 1.8 gurea was added to the mixture in the ratio Zn:urea=1:2 (molar ratio).After stirring for 1 h, the solution was transferred to an autoclave andthe reaction mixture was purged with 7500 torr Ar 5 times, and then apressure of 7500 torr Ar was imposed before initiating heating. Themixture was heated to 200° C. for 5 h, then to 265° C. and maintained atthat temperature for 1.5 h; finally, the vapor inside was vented. Afterthe supercritical fluid drying (SCFD), a green powder was collected andsubsequently calcined with a ramp rate of 3° C./min to 500° C., thenmaintained at 500° C. for 6 h. The powder produced from this preparationcontains numerous 6-fold building rods or prisms aggregates of holesrich ZnO structures. The aggregates were flower-like, but less “open”than the flower-like aggregates of the previous two examples. Thetypical diameter of these building rods or prisms is about 0.3-6 μm, andthe lengths of the rods or prisms are about 0.3-2 μm.

Preparation of flower-bud ZnO aggregates 18 g of Zn(NO₃)₂.6H₂O wasdissolved in 200 ml absolute methanol. After the Zn(NO₃)₂.6H₂O dissolvedcompletely, 13 g benzyl alcohol was added to the mixture in the ratioZn:BA=1:2 (molar ratio). After stirring for 1 h, the solution wastransferred to an autoclave and the reaction mixture was purged with7500 torr Ar 5 times, and then a pressure of 7500 torr Ar was imposedbefore initiating heating. The mixture was heated to 200° C. for 5 h,then to 265° C. and maintained at that temperature for 1.5 h; finally,the vapor inside was vented. After the supercritical fluid drying(SCFD), a grey powder was collected and subsequently calcined in airwith a ramp rate of 3° C./min to 500° C., then maintained at 500° C. for6 h. The powder produced from this preparation contains numerousflower-bud aggregates of holes rich ZnO structures.

Preparation of Layered Column Structures: 18 g of Zn(NO₃)₂.6H₂O wasdissolved in 200 ml absolute methanol. After the Zn(NO₃)₂.6H₂O dissolvedcompletely, 0.9 g urea and 39 g benzyl alcohol was added to the mixturein the ratio Zn:urea:BA=1:0.5:6 (molar ratio). After stirring for 1 h,the solution was transferred to an autoclave and the reaction mixturewas purged with 7500 torr Ar 5 times, and then a pressure of 7500 torrAr was imposed before initiating heating. The mixture was heated to 200°C. for 5 h, then to 265° C. and maintained at that temperature for 1.5h; finally, the vapor inside was vented. After the supercritical fluiddrying (SCFD), a grey powder was collected and subsequently calcinedwith a ramp rate of 3° C./min to 500° C., then maintained at 500° C. for6 h. The powder produced from this preparation contains numerous layeredcolumns aggregates of holes rich ZnO structures. The typical diameter ofthese columns is about 2-6 μm, the thickness of layers is about 50-100nm, and the lengths of columns are about 1-4 μm.

Instrumentation: The materials were characterized by powder X-raydiffraction (XRD) using a Siemens D500 X-ray diffractometer with nickelfiltered Cu Kα radiation (λ=1.5418 Å) at a scanning rate of 0.1° .min⁻¹in the 2θ range of 10-80°.

Field emission scanning electron microscopic characterization of thesamples was carried out on a JEOL JSM-7000F. Transmission electronmicroscopic and electron diffraction characterization of the sampleswere carried out on a JEM-2010 operated at 200 kV.

Transmission electron microscopic (TEM) characterization of theflower-like ZnO samples was carried out on a JEM-2010 operated at 200kV. The samples were prepared by spreading an ultrasonicated suspensionin ethanol.

N₂ adsorption-desorption isotherms were obtained using a Micrometric'sASAP 2020. The samples were degassed at 300° C. in vacuum for more than4 h prior to the measurement. The specific surface areas were evaluatedwith the Brunauer-Emmett-Teller (BET) method in the P/P₀ range of0.05-0.35. Pore size distributions were calculated from the adsorptionbranch of the isotherms with the Barrett-Joyner-Halenda (BJH) method,and pore sizes were obtained from the peak positions of the distributioncurves.

FIG. 11, trace a) shows the powder x-ray diffraction (XRD) pattern ofthe flowerlike ZnO nanostructures calcined at 500° C. The sample wassynthesized when these reactants were in the ratio ofZn(NO₃)₂.6H₂O:urea:benzyl alcohol=1:0.5:2 (molar ratio). The peaks at28=31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.8°, 66.4°, 67.9°, 69.1° areobserved from FIG. 11( a). According to standard ZnO XRD pattern (JCPDScard no. 36-1451) shown in FIG. 11( b), the products are hexagonal ZnOphases (a=0.325 nm, b=0.325 nm, c=0.521 nm), and these peaks areassigned to (100), (002), (101), (102), (110), (103), (200), (112), and(201) diffraction lines of hexagonal ZnO phases, respectively. Thisindicates that the ZnO is a single phase of well-crystallized ZnO withthe hexagonal wurtzite structure (space group: P63mc(186)). The specificsurface area measured with the BET technique is ca. 2 m²/g.

Field emission scanning electron microscope (FESEM) images show themorphology of the flower-like ZnO structures before and aftercalcination at different magnifications. The as-synthesised productshows a flower-like structure (FIG. 1). After calcination, the ZnOproduct contains numerous flower-like aggregates (FIGS. 2 a-2 d). Theseflowers were composed of 6-fold building blocks, as shown in the FESEMimage in FIGS. 2 b, 2 c and 2 d at different magnification. Thediameters of these 6-fold building blocks are ca. 1-3 μm, and thelengths are ca. 2-6 μm. It is notable that many hole or pit-like surfacefeatures are also observed in these 6-fold building blocks from FIGS. 3a and 3 b.

High resolution transmission electron microscope (TEM) images reveal themorphology and structure of these 6-fold building blocks (FIG. 4). FIG.4 a reveals the morphology of one 6-fold building block with holeystructure. FIG. 4 b shows the TEM of the 6-fold building block. The FFTof selected area can be indexed as the [0110] zone axis ofsingle-crystalline ZnO with a hexagonal structure. A representativeHRTEM image taken from the 6-fold building block is shown in FIG. 4 c.The lattice fringes are clearly visible with a spacing of 0.26, 0.16 and0.14 nm. The spacing of 0.26 nm corresponds to the lattice fringes ofthe (0002) planes, the spacing of 0.16 nm corresponds to the latticefringes of the (2 1 10) planes, and the spacing of 0.14 nm correspondsto the lattice fringes of the (2 1 12) planes, respectively. The FFTs(inset) also reveals that the crystal structure is wurtzite oriented toZ=[01 10]. FIGS. 5 a-b and 6 a-b show TEM images of a ZnO nanorod of theleft and the right edges of the 2°. It indicates the growth direction ofthe nanorod corresponds to {0001}. Selected Area Electron Diffraction(SAED) (FIG. 7 b, FIG. 8 a-c) also shows that the nanorod presenthexagonal crystal structure oriented to the [01 10] zone axis. FIG. 9 ashows nine different nanorods were analysed by EBSD. All of themexhibited the same wurtzite crystal structure. For example, FIG. 9 b isrepresentative raw indexed electron back-scatter pattern (EBSP)s fromthe nanorod labeled 3 in the adjacent SEM image. The crystal structureis found to be wurtzite, with [01 10] orientation. We know from the SAED(FIG. 10) that the edges of the nanorod tip correspond to (2 1 12) and (2 112) planes. The measured angle results to be 65° close to thetheoretical value of 64.42°. The edges along the length are ( 2 110)planes. From the crystal structure, ZnO is wurtzite structure whichconsists of polar (0001), (0001) planes and non-polar (1000) planes withC6v symmetry. Due to its anisotropic crystal structure, the c-axis isthe most preferred growth orientation, and the velocities of growth indifferent directions under hydrothermal condition areV[0001]>V[0110]>V[1000] (Wang, M., et al., J. Cryst. Growth 2006, 291,334). The holes indicate the defects in the single-crystalline ZnOstructures.

From the SEM images and EBSD data, in combination with TEM images andselected area electron diffraction analyses, the morphology and surfacestructure of the ZnO flowers were determined as shown in FIG. 15. Theimages show that the flowers are made up of hexagonal prisms withpyramidal tips. The prisms are bounded by non-polar {01 10} planes withintersecting edges along {2110} planes while the pyramids are composedof { 1101} surfaces with intersecting edges along {2112} planes. Withoutwishing to be bound by any particular belief, one explanation for theenhanced catalytic activity observed in these flower structures is thecatalytic activity of the {−1101} surfaces of the pyramids and the{2112} edges.

Within this work, several experiments have been carried out to determinethe parameters that are important for the formation of ZnO withdifferent shapes and sizes. In the absence of benzyl alcohol and urea,ZnO with the irregular shape were prepared. When the ratio ofZn(NO₃)₂.6H₂O:benzyl alcohol=1:2 (molar ratio) is a constant, the sizeof ZnO 6-fold building rods decreases with decreasing the amount ofurea. Smaller ZnO 6-fold building rods with diameter ca. 1 μm wereobtained when the ratio of Zn(NO₃)₂.6H₂O:urea=1:0.25, the aggregatesstill look like ‘blooming flowers’. In the absence of urea, the lengthand diameter of ZnO 6-fold building rods decreases considerably, and theaggregates of these smaller ZnO 6-fold building rods look like ‘buddingflowers’ as shown in FIG. 16. According to the experimental results, therole of urea is important to control the size of the ZnO 6-fold buildingrods and the shape of aggregates in the synthesis method as it providesa steady OH⁻ supply via urea hydrolysis (Ref. 7). When Zn(NO₃)₂.6H₂Oreacts with methanol and water to form the ZnO precursor, acid is aby-product, and the accumulation of acid will inhibit the furtherformation of the ZnO precursor. However, when urea is added, the OH⁻formed by urea hydrolysis neutralizes the acid and allows the formationof the ZnO precursor. Thus, in the self-assembly process, we suggestthat the steady OH⁻ supply controls the size of ZnO building rods bymoderating the hydrolysis and alcoholysis rates of zinc nitrate.

Here, benzyl alcohol is used as a structure-directing agent to controlthe synthesis of ZnO structures. The effect of benzyl alcohol was alsoinvestigated. In the absence of benzyl alcohol, although 6-fold buildingrods or prisms were obtained; the length and diameter are inhomogeneous.Layered columns were formed when the molar ratio ofZn(NO₃)₂.6H₂O:urea:benzyl alcohol=1:0.5:6. Thus, since all otherparameters were unchanged, it can be inferred that benzyl alcohol playsa critical role as a structure-directing agent to control the shape ofthese 6-fold building rods. In previous literature, most theoriesregarding the role of organic compounds are that the organic compoundsact as a) simple physical compartments, b) to control nucleation or c)to terminate crystal growth by surface poisoning through selectiveadsorption on certain planes (See Refs. 4d,4e,8). In this case, largeamounts of benzyl alcohol lead to decreasing length of 6-fold ZnO rods,and forms these layered ZnO columns and infers that benzyl alcohol mayadsorb on (002) planes to terminate the crystal growth. There are manyholes in all these ZnO samples (as observed in electron microscopyimages) prepared by different methods, and Wang and coworkers reportedthe preparation of ZnO rings from ZnO disks; they thought that theformation of a hole could be due to a high density of defects at thecenter of the ZnO disks that resulted in a high local reaction/etchingrate under heating (See Ref. 9). In the case of the present work, asimilar hole formation mechanism may occur. The high reaction/etchingrate at the defect sites in the ZnO rods may lead to the formation ofholes. XRD results showed that all ZnO samples are single phase ofwell-crystallized ZnO with the hexagonal wurtzite structure.

EXAMPLE 2 Methanol Dissociation and Carbon Dioxide Hydrogenation

In-situ DRIFTS investigation: A Thermo 6700 IR spectrometer with liquidnitrogen cooled detector, a high temperature environmental chamber andDRIFT accessory were used with the following parameters: 64 scans,600-4000 cm⁻¹ scan range, 4 cm⁻¹ resolution.

In-situ DRIFTS investigation of methanol adsorption and surfacereaction: The sample temperature was measured through a thermocoupleinserted into the sample holder directly in contact with the sample. 10mg of sample was placed into a high temperature environmental sampleholder; the chamber was heated to 500° C. under nitrogen flow and keptfor 2 hours, then the temperature was decreased to room temperature. Aspectrum of the ZnO sample was collected at room temperature undernitrogen and was used as the background. Then the sample was exposed tomethanol vapor for 2 min by nitrogen at a flow rate of 100 ml per min.The spectra were collected under nitrogen at room temperature. Followingthis scan, the temperature was raised to 70° C., and another scan wastaken, and kept at 70° C. for half an hour, another scan was taken.After this scan, oxygen was introduced at 70° C. and maintained for halfan hour, finally, the last spectra was collected at 5 min and half anhour, respectively.

In-situ DRIFTS investigation of CO₂ hydrogenation: 10 mg of sample wasplaced into a high temperature sample holder; the chamber was heated to500° C. under nitrogen flow and kept for 2 hours, then the temperaturewas decreased to 180° C. A spectrum of the ZnO sample was collected at180° C. under nitrogen and was used as the background. Then the mixtureof CO₂ and H₂ (molar ratio of CO₂ and H₂ is 1:3) was introduced into thechamber at 1 atm, the spectra were collected at different timeintervals.

Defects at metal oxide surfaces are thought to significantly influence avariety of surface properties, including chemical reactivity (Li, F., etal., Angew. Chem. Int. Ed. 2004, 43, 5238). Methanol is a ‘smart’molecular probe that can provide fundamental information about thenumber and the nature of active surface sites (BASF., German Patents,1923, 415, 686, 441, 443, 462, and 837, US Patents, 1923, U.S. Pat. Nos.1,558,559 and 1,569,755; Sun, Q., et al., J. Catal. 1997, 167, 92 andOlah, G. A. Angew. Chem. Int. Ed. 2005, 44, 2636). Methanoldecomposition has been found to be structure sensitive, in that theselectivity depends on the arrangement of the surface atoms. Methanolalso can be an interesting combustible for fuel cells. We investigatedmethanol adsorption and reaction on the surface of ZnO structures bydiffuse reflectance infrared Fourier transform (DRIFT) spectroscopictechniques at low temperatures. DRIFT spectra of the flowerlike singlecrystalline ZnO exposed to methanol vapor introduced by nitrogen flow atroom temperature was collected (FIG. 12). At room temperature, methanolinteracts molecularly and dissociatively with flowerlike singlecrystalline ZnO. The molecular interactions are indicated by the C—Ostretching peak centered at 1031 cm⁻¹ and the corresponding symmetricand asymmetric C—H stretching contributions at 2845 and 2950 cm⁻¹. Thedissociative interactions are indicated by the C—O stretching region1054 and 1014 cm⁻¹, and the presence of methoxyl groups is confirmed bythe signals corresponding to the symmetric and asymmetric C—H stretchingcentered at 2819 and 2922 cm⁻¹ (Natile, M. M., et al., Chem. Mater.2006, 18, 3270 and Natile, M. M., et al., Chem. Mater. 2005, 17, 3403).It is worth noting that carbon monoxide as indicated by the peaks at2070 and 2040 cm⁻¹ was observed on the surface of flowerlike singlecrystalline ZnO. This indicated that methanol can be oxidized intocarbon monoxide in absence of oxygen on the surface of flowerlike singlecrystalline ZnO at room temperature. Methanol dissociation and oxidationon the surface of flowerlike single crystalline ZnO is favored by theincreasing temperature. When the temperature was increased to 70° C.under nitrogen, methanol was decomposed partly into carbon dioxide asindicated by the peaks at 2362 and 2314 cm⁻¹ and carbon monoxidedisappeared (FIG. 13 b). Methanol was further oxidized a little after itwas kept at 70° C. for half an hour (FIG. 13 c). When oxygen by amixture of oxygen and nitrogen (20.5% oxygen and 79.5% nitrogen) wasintroduced into the in-situ chamber, and kept for 5 min, methanol wasfurther oxidized as indicated by the peaks increasing centered at 2362and 2314 cm⁻¹ and the decreasing of peaks in the C—H stretching regionbetween 2800 and 3000 cm⁻¹ and the peaks in the C—O stretching regioncentered at 1054, 1031 and 1014 cm⁻¹ (FIG. 13 d). After half an hour,methanol was almost oxidized completely (FIG. 13 e).

For comparison, DRIFT spectra were also collected for NanoActive ZnOwith high surface area (from NanoScale Corporation, 70 m²/g andcrystallite size 10 nm) exposed to methanol vapor introduced by nitrogenflow at room temperature. At room temperature, methanol interactedmolecularly and dissociatively with the NanoActive ZnO. It is worthnoting that peaks at 2070 and 2040 cm⁻¹, corresponding to carbonmonoxide, were not detected for the room temperature experiment withNanoActive ZnO. In addition, when the same higher temperatureexperiments described in the previous paragraph were done on the surfaceof NanoActive ZnO, neither carbon dioxide nor carbon monoxide wasobserved under nitrogen even at 70° C. for half an hour. After oxygenwas introduced, methanol was oxidized and carbon dioxide was observed asindicated by the peaks at 2362 and 2314 cm⁻¹. Methanol can be furtheroxidized partly by oxygen with increasing reaction time on the surfaceof NanoActive ZnO. After the adsorption and reaction of methanol, thecolor of flowerlike single crystalline ZnO did not change, but theNanoActive ZnO became black. This indicated that carbon deposited on thesurface of NanoActive ZnO, but on the surface of the flowerlike singlecrystalline ZnO, methanol was completely oxidized into carbon dioxide.

Carbon dioxide has become the focus of attention recently because it isa major greenhouse gas and a cheap C1 resource. The conversion of CO₂into value-added chemicals has also attracted much attention in recentyears (Jessop, P. G., et al., Chem. Rev. 1995, 95, 259; Leitner, W.Angew. Chem. Int. Ed. 1995, 34, 2207; Shaikh, A. A., et al., Chem. Rev.1996, 96, 951; Gibson, D. H. Chem. Rev. 1996, 96, 2063 and Yu, K. M. K.,et al., J. Am. Chem. Soc. 2007, 129, 6360). The hydrogenation of CO₂ toproduce formic acid or methanol is an attractive reaction system(Jessop, P. G., et al., Nature 1994, 368, 231 and Munshi, P., et al., J.Am. Chem. Soc. 2002, 124, 7963). CO₂ hydrogenation is run mostly onCu/ZnO catalysts at temperatures of 220-280° C. (BASF., German Patents,1923, 415, 686, 441, 443, 462, and 837, US Patents, 1923, U.S. Pat. Nos.1,558,559 and 1,569,755; Sun, Q. et al., J. Catal. 1997, 167, 92 andZhang, Z., et al., Angew. Chem. Int. Ed. 2008, 47, 1127).

Surprisingly, the Cu-free flowerlike ZnO showed high activity for CO₂hydrogenation as shown in FIG. 14, which shows DRIFT spectra obtainedafter exposing the flowerlike single crystalline ZnO to CO₂ and H₂ at180° C. for 1 min (a), 10 min (b), 20 min (c), 40 min(d), 60 min (e). At180° C. for 10 min, the C—H stretching bands were observed at 2982, 2970and 2881 cm⁻¹, in the meantime, two types of C═O stretching bands at1740 and 1711 cm⁻¹ which can attributed to adsorbed formate andformaldehyde, repectively; the bands at 1516, 1368, 1214 cm⁻¹ which canattribute to adsorbed formic acid and the band at 1048 cm⁻¹ which can beattributed to C—O stretching of adsorbed methoxyl group (Schilke, T. C.,et al., J. Catal. 1999, 184, 144 and Jung, K. D., et al., J. Catal.2000, 193, 207). These results indicate that the products of CO₂hydrogenation include formic acid, formaldehyde and methanol. Thestrength of these peaks increased with the increasing of reaction time.These results indicate that the flowerlike single crystalline ZnO showedgood surface activity. Without wishing to be bound by any particularbelief, the surface activity may be due to the presence of large amountof holes (defect sites), the crystallographic orientation at the tip ofthe ZnO building blocks, or a combination thereof.

For comparison, hydrogenation of CO₂ was not observed over NanoActiveZnO with high surface area. Only hydrogen and CO₂ adsorption, which areindicated by the peaks in the range of 3500-3900 cm-1 and in the rangeof 1000-1700 cm-1, respectively, were observed.

We claim:
 1. A method for making a zinc oxide structure, the methodcomprising the steps of: a. preparing a mixture of i. a zinc salt or ahydrated zinc salt, the zinc salt being selected from the groupconsisting of zinc nitrate, zinc acetate, zinc citrate, zincmethacrylate, zinc sulfate, zinc oxalate and combinations thereof; ii. afirst additive, the first additive being an alcohol comprising a phenylgroup; iii. a second additive selected from the group consisting ofurea, a urea derivative, thiourea, a thiorurea derivative andcombinations thereof; and iv. an aliphatic alcohol solvent, thealiphatic alcohol having from 1 to 3 carbon atoms; b. heating themixture of step a) to a first temperature from 180° C. to 200° C. andmaintaining the mixture at the first temperature for a time from 1 to 12hours; c. heating the mixture of step b) to a second temperature from240° C. to 300° C. and maintaining the mixture at the second temperaturefor a time from 1 to 12 hours; d. removing the alcohol solvent from themixture of step c) and e. calcining the mixture of step d) in air. 2.The method of claim 1, wherein the first additive is benzyl alcohol or a4-substituted benzyl alcohol.
 3. The method of claim 2, wherein themolar ratio of benzyl alcohol or substituted benzyl alcohol to Zn isfrom 2 to
 6. 4. The method of claim 1, wherein the second additive isurea.
 5. The method of claim 1, wherein the molar ratio of urea to Zn isfrom 0.25 to 0.5.
 6. The method of claim 3 wherein the molar ratio ofurea to Zn is from 0.25 to 0.5.
 7. The method of claim 1, wherein thealcohol solvent is methanol, ethanol or propyl alcohol and theconcentration of zinc ion in the solution is from 0.01 mol/l to 1 mol/l.8. The method of claim 7, wherein the alcohol solvent is methanol. 9.The method of claim 1, wherein the mixture of step a) comprises ahydrated zinc salt.
 10. The method of claim 9, wherein the hydrated zincsalt is zinc nitrate hexahydrate.